Zinc pencils and brass plugs are often used, for example, in marine engines to help reduce or prevent the effects of electrolysis. Typical prior art brass plugs, used with zinc pencils, have outer threads and a cavity with inner threads, which are threaded in the same direction as the outer threads. During operation the zinc pencil will corrode, expand and form a crust on the outside of the zinc pencil. This makes the pencil diameter larger then the cavity of the engine into which the pencil was placed, making it difficult to remove the pencil from the engine cavity.
To remove a zinc pencil from an engine cavity one would turn the plug in a counter clockwise direction (for example) to unscrew the plug from the engine cavity. However, when the zinc pencil is corroded the zinc pencil will not move easily and turning the plug in the counter clockwise direction while the zinc pencil is not moving will cause the pencil to unscrew from the plug. This is because the outer threads on the plug and the inner threads in the cavity of the plug are threaded in the same direction.
If the pencil unscrews from the plug while the pencil is in the engine cavity, the pencil may fall into the cooling system of the engine. This can cause various undesirable results including restricted water flow in the engine.